Make sure to scare him a little too.
Remind him of the seriousness of the law,
that he should never smoke where it's unsafe and never be caught with it,
and one very important thing that most parents seem to miss:
Just because you both smoke cannabis, does not mean other drugs are ok as well.
You NEED to explain to him that cannabis is relatively safe, but about 95 percent of all other drugs are unsafe, or very very very unsafe.
Tell him not to be afraid to talk to you about anything.
I was 15 once too, blah blah blah, please don't lie or steal, be direct and honest and respectful and I will be as well, and maybe when you get older, we can do this together sometime.
Hope it goes well.
Try not to be too intimidating.